The present disclosure involves the creation of several charged species by a pulsed DC spark discharge acting on a carrier gas, typically helium, which utilizes the charged species to classify and/or quantify compounds in the carrier. This detector is connected with upstream or downstream devices such as a sample source, gas chromatograph (GC) column spectrum analyzers, etc. Understanding of various test procedures will illuminate use of the described apparatus and can be gained from review of the apparatus and its mode of operation in a system. A sample to be evaluated is first loaded along with a carrier gas into a system column. The sample passes through this device, a pulsed, high voltage, direct current (DC) spark discharge which forms selected charged or energized species as will be described. As a result of the spark discharge, several types of detection systems are initiated by this detector. For instance, the very short DC spark creates a readily available thermalized electron flux which can be used in a detection system. In an alternate mode of operation, the spark also creates a more slowly diffused flux of metastable helium atoms which drift toward selected electrodes at a controlled rate. The helium atoms will react with molecules of the sample to surrender the excess energy from the excited state to cause sample molecule ionization which, as a secondary reaction, can be measured by a detection system. Another aspect involves transitory photoionization of gas into positive and negative charged particles normally recombining at high speed. If a selected sweep pulse voltage is applied, the recombination is prevented to furnish a current signal indicative of gas contents.
The preferred form this system features a pulsed DC spark discharge in the carrier gas flow which is followed by a rather slow metastable carrier gas dispersion and secondary reaction, which is slow in contrast with the practically instantaneous electron initiated interaction. The DC spark discharge therefore enables two different detection mechanisms, as will be explained, so that variations in detection electrode geometry and pulse timing can obtain different types of responses. One system uses the highly mobile electron flux while an alternate system relies on the metastable carrier gas molecular energy interchange occurring well after the electron flux.
In addition to the particle interaction initiated in the spark manifest in two different aspects, there are also two spectral emissions created by the DC spark, one occurring during the spark and the other occurring later. In the spark gap, the electron discharge creates a first observable spectrum which can be observed by viewing the spark region. Geometry of the spark is sharply defined, narrowly confined, and repetitively located for observation and spectral analysis. This analysis enables detection of the atomic species in the gap. While this first spectrum is extinguished at the end of the spark, a second spectral analysis is enabled by the subsequent decay of metastable helium atoms giving up their excess energy by ionizing molecules of the sample. This interchange occurs as the energized helium atoms diffuse from the spark gap in the test chamber and with the sample molecules. Dependent on relative concentrations, diffusion and flow rates, the sample molecules are ionized to emit energy characteristic of the species. This delayed emission is useful in species identification when timely observed, and therefore a different mode of observation is used to capture data from this emission. This difference in operation derives primarily from delayed occurrence and observed at a different time.
The present invention uses to advantage a simple spark gap having a pair of spaced electrodes connected to a current pulse forming system. The pulses are extremely narrow, preferably in the range of a fraction of a microsecond. The DC pulses repetitively form precise, sharp and well defined transgap pulses, liberating the electron flux mentioned and also forming the metastable helium molecules. The spark is fixed in size and relative timing, shape and location. Electrode geometry does not erode with time and electron ejection is uniform. Thus, the spark is fixed for observation by spectral analysis. Structurally, this enables a very simple chamber to deploy a pair of opposing, spaced electrodes in a cavity or perhaps 10 to 100 microliters volume with gas flow inlet and outlet ports. In a representative system, a chemical sample is mixed with a carrier gas. The sample is prepared for testing by classification, identification or quantification using the detector. An exemplary system achieves separation as a result of differences in travel time through a GC column input to the detector. As is well known, the GC column is packed with a stationary phase material so that the carrier gas and the compounds making up the sample are eluted from the GC column. As a generalization, the mobile phase (a flowing gas or liquid) is delivered by the GC column into this detector for detection of the separated chemical constituents making up the sample.
The detector is operated periodically to test each of the sample constituent compounds passing through the detector. One type of detector used in the past has been the electron capture detector (ECD). The present disclosure sets out an alternate form of ECD detector used in conjunction with a GC column which forms an output signal of substantial sensitivity. The present system features an ECD with a DC pulsed, high voltage spark discharge. As noted at column 2 of U.S. Pat. No. 4,851,683, DC discharges have been known, but they generally have had inhomogenous excitation characteristics and are unstable because of electrode heating and erosion. U.S. Pat. No. 4,509,855 is a DC atmospheric pressure helium plasma emission spectrometer. Additional devices are shown in U.S. Pat. No. 4,866,278. The present apparatus sets forth a DC pulsed, high voltage, spark discharge source which provides a repetitive uniform spark. The spark has a duration which is only a fraction of a microsecond. It would appear that an acceptable spark duration is a part of a microsecond. Moreover, the spark gap is structurally fixed to have a finite width for discharge of the spark created by accumulating energy in a reactive circuit such as a coil and capacitor tank circuit and dumping the energy across the spark gap after charging. Preferably, a nonringing current is applied.
This detector in a representative form includes a means for forming a stabilized spark gap so that the spark and resultant charged particle population are uniform in contrast with the problems referenced in the two mentioned patents. Accordingly, the carrier fluid (e.g., carrier flow from the GC column) is directed as a gas flow through appropriate tubing into a spark chamber. The gas is forced to flow in the spark chamber past a pair of electrodes which are arranged to direct the spark transverse to the gas flow. In a first mode of operation, a flux of electrons is obtained. These electrons are quickly dissipated during the spark interval even though spark duration is only a fraction of a microsecond. The number of electrons available can be measured by means of an electrometer connected to electrodes spaced remotely from the spark gap. The circuitry connected with a terminal spaced from the spark gap detects and measures the electron flux resulting from the spark discharge. In this instance, the spark gas works as an ECD. There is, however, an alternate charged particle flux which is delayed after the spark discharge which uses an ionization mode. This involves a delay of up to about 100 or even 200 microseconds after the spark discharge creates ionized molecules which are dispersed at a slower rate compared with the more mobile electron dispersal. The spark disperses highly energized electrons during the spark and also creates a second and slower dispersion of metastable carrier gas molecules (preferably helium) after the spark. Charged particle dispersal of the first form is, as a practical matter, instantaneous while metastable helium dispersal is time delayed. The two types of dispersion are readily identified because they involve different types of particles. The dispersal of metastable helium atoms, with an elevated energy state of about twenty or more eV, can be observed at a distance from the spark gap so that sample compound concentration (a scale factor) in the chamber is measured. The metastable helium concentration is useful because it enables this delayed reaction. Thus, the metastable helium atom reacts with the sample molecules input with the carrier flow. The high energy in the helium ionizes the sample molecules, creating a measurable current in the chamber.
Building on the last possibility, metastable helium molecules may combine with a trace constituent such as a dopant supplied with the carrier (helium) gas. One such dopant is nitrogen which, in reaction with the metastable helium, forms nitrogen ions. That causes liberation of electrons which again, because of different mobility, dissipate more readily. Before the electrons recombine with the ionized nitrogen molecules, they will react with the compounds making up the sample flowing through the detector. A connected electrode and electrometer will measure electron capture from the dopant involvement to define an electron capture detector.
Another alternate form of apparatus involves observation of the spectrum emitted in the spark gap. This involves the conversion of the constituents to elevated energy states where emissions occur at characteristic frequencies, and such frequencies can be observed and measured. This typically involves a spectrum analyzer such as a spectrometer which observes one or more atomic or molecular emission lines in selected regions of the spectrum. Spectral line observation varies with the time and location relative to the spark discharge. Regarding time, the observed spectrum is different during and after the spark discharge. Regarding location, the reaction is different in the spark or elsewhere in the chamber. The present apparatus is therefore summarized as a pulsed DC spark discharge where the spark discharge reacts with a carrier gas (preferably helium) and compounds from a sample. In this spark, charged particles are created, the particles being either disassociated electrons, an ionized carrier gas, ionized dopant gas, or highly energized helium atoms in a metastable form. Depending on the timing of measurements, the particular ionized particles and measurement voltages applied, the device can be operated in an ionization mode, or electron capture mode. Molecules of a compound separated by chromatographic separation or other input devices can be quantified. The device also emits characteristic spectral lines depending on the nature and timing of the emission. Moreover, by selection of the carrier gas, the addition of a selected dopant with the gas flow, control of pulsing of the spark gap forming the charged particles, timed operation of measurement electrodes nearby, and adjustment of scale factors, it is possible to operate in several modes. In addition to this, precisely defined spectral lines can be observed.